Microelectronic and optoelectronic components are now made from combinations of monocrystalline materials manufactured using epitaxy techniques. Modern epitaxy techniques make it possible to prepare and combine materials of very high purity having properties close to their ideal theoretical performances.
However, a certain number of technological difficulties remain, hindering the full development of these techniques. Indeed, in the case of optoelectronic applications, semi-conductor alloys cover a very wide range of composition and thus of associated wavelengths and applications. However, the monocrystalline nature of the substrates and the use of epitaxy techniques lead to intrinsic restrictive limitations, since it will be possible á priori to only combine materials having identical or very similar lattice parameters. Indeed, as a function of the substrates available, only narrow ranges of composition may be exploited.
Attempts have been made to grow, by heteroepitaxy, alloys having a lattice mismatch on standard substrate. Said alloys are then strained. Beyond a thickness known as critical (of several nm), the strain energy causes the plastic relaxation of the epitaxied layer. This results in the appearance of extensive structural defects (low mismatch) or island growth (high mismatch). This mode of relaxation leads to a roughening of the surface and to a deterioration in the optical and electronical properties of the materials.
The document of Y. H. Lo et al., Appl. Phys. Lett. 59, 2311, 1991 proposes the formation of a lattice parameter mismatched layer. It involves a strained layer of thickness less than the critical plastic relaxation thickness, which is elastically relaxed to avoid generating defects. A resumption of lattice parameter matched epitaxy may then be carried out on this relaxed layer, over large thicknesses, without generating defects. This concept makes it possible to obtain layers of compositions different to the compositions used on standard substrate.
This approach consists in epitaxying a strained layer on a substrate, then relaxing said layer to allow a resumption of epitaxy without generating structural defects. The relaxation can take place before or during the resumption of epitaxy.
This relaxation may be obtained by different methods, such as the use of a viscous material or sub-etching.
This approach has been applied by Yin H. Yin et al., J. Appl. Phys. 91, 9716, 2002, with the use of a glass (BPSG), as well as by M. Kostrzewa et al., J. Cryst. Growth. 275, 157, 2005, with the use of a wax. In this technique, the structure is transferred onto a viscous material. The strained layer, once released from the substrate on which it has been epitaxied, relaxes thanks to the viscosity of the support substrate, generally by heating the viscous material in order to increase its viscosity.
However, in these techniques, the viscosity of the layer is not high enough to obtain good relaxation. In fact, this takes place through formation of surface undulations, which can have major drawbacks. As a function of the amplitude and the wavelength of the undulations, it can lead to a surface roughening of the epitaxial layer on account of the non-uniform deformation field that it generates on the surface of the material. Techniques employing transfer on viscous material are thus limited.
The sub-etching technique applied by Damlencourt et al., Appl. Phys. Lett., 75, 3638, 1999, consists in forming an almost free strained layer held by arms. The layer is released from its support by sub-etching of a stop layer. The layer is then free to relax before it comes to bond on the initial substrate. However, being held by arms limits the relaxation and the selectivity of the strained layer with respect to the stop layer and limits the dimension of the relaxed membranes.
It is thus necessary to develop a novel technique enabling the relaxation of layers, particularly of large dimensions, without the above mentioned limitations.